1. Field of the Invention
This invention relates to a fuel metering control system for an internal combustion engine.
2. Description of the Related Art
The PID control law is ordinarily used for fuel metering control for internal combustion engines. The control error between the desired value and the controlled variable (plant output) is multiplied by a P term (proportional term), an I term (integral term) and a D term (differential or derivative term) to obtain the feedback correction coefficient (feedback gain). In addition, it has recently been proposed to obtain the feedback correction coefficient by use of modern control theory or the like, as taught by Japanese Laid-Open Patent Application Hei 4(1992)-209,940.
When conducting feedback control using a controller such as the adaptive controller, during a fuel cutoff, the exhaust air/fuel ratio should substantially be zero, since the supply of fuel is shut off and no combustion occurs. As the limit of the measurable range of the air/fuel sensor in the lean direction is approximately 30: 1, however, this state is beyond the limit, and it is impossible in practice to accurately detect the air/fuel ratio under such a no fuel supply state.
Accordingly, it is not possible to continue the adaptive control with properly calculated controller internal variables during the fuel cutoff, since the controller internal variables must be determined in response to the detected air/fuel ratio. Therefore, it is difficult to start the adaptive controller to properly operate immediately after resumption of the fuel supply following the termination of the fuel cutoff. This degrades the convergence rate or speed of control and hence control performance.